Along with the recent increase in the capacity of a magnetic disk apparatus, the recording density and the track density are also rising. When the track density becomes higher, the interval (that is, track pitch) between neighboring tracks on the disk narrows.
With the narrower track pitch, when magnetically writing data on a track, for example, data (magnetic data) already magnetically written on a neighboring track may suffer magnetic degradation. That is, the magnetic data (more specifically, the strength of the magnetic field of the magnetic data) on the neighboring track may weaken. This phenomenon is called a side-erase effect. The side-erase effect occurs because of an error upon aligning the head with a target track and a magnetic field that leaks from the head.
Even when the magnetic data of the corresponding track weakens because of the side-erase effect, no read error occurs immediately. When the side-erase effect repetitively occurs, the magnetic data of the corresponding track gradually weakens. If the number of times of side-erase effect occurrence exceeds a predetermined threshold, it is difficult to recover the magnetic data by an attempt using error correction codes (ECCs) to the maximum, resulting in a read error. As a result, the magnetic data is lost.
In a recent magnetic disk apparatus, data refresh is becoming indispensable to recover magnetic degradation of magnetic data before reading the magnetic data from the track becomes impossible because of the repetitive side-erase effect. Data refresh is known as an operation of reading magnetically degraded magnetic data from the corresponding track and writing the read magnetic data again to the track. The rewrite (that is, data refresh) of the magnetic data strengthens and stabilizes magnetic field of the magnetic data.
The track where the side-erase effect repetitively occurs and the number of data writing to a track adjacent to that track have correlation. A conventional magnetic disk apparatus uses a technique of counting the write count for each track or each group on the disk to use the above-described correlation. This technique refreshes data of a track adjacent to a track where the write count has reached a threshold or data of all tracks belonging to a group where the write count has reached a threshold.
In the above-described related art, a memory such as a dynamic RAM (DRAM) is used to hold the write count of each track or each group (that is, for a predetermined recording unit). In general, the smaller the recording unit to count the write count is, the larger the memory capacity necessary for holding the write count is. On the other hand, the larger the recording unit to count the write count is, the larger the number of times of unnecessary data refresh is.